The present invention relates to a high-performance, R--Fe--B-based, sintered permanent magnet, wherein R is one or more of rare earth elements including Y, made of a coarse alloy powder prepared by a reductive diffusion method. The present invention also relates to a method for producing the R--Fe--B-based, sintered permanent magnet.
An R--Fe--B-based, sintered permanent magnet may be typically produced by a metallurgical method including the steps of melting and casting metals for the magnet to form an alloy ingot, pulverizing the ingot to alloy powder, molding and sintering the alloy powder, heat-treating the sintered body and then working it. Since the rare earth metals are extremely expensive, efforts have been directed toward reducing the production cost of the permanent rare earth magnet.
Japanese Patent Laid-Open No. 59-219404 proposes a so-called reductive diffusion method for preparing a coarse alloy powder for producing a rare earth permanent magnet. In this method, cheep rare earth oxides in the starting material are reduced by a reducing agent such as metallic calcium, metallic magnesium, etc. to rare earth elements which diffuse into the other alloying metals in the starting material. Since the reduction of the rare earth oxides is accompanied by by-production of CaO, MgO, etc., the by-produced oxide should be removed from the coarse alloy powder, because they are detrimental to magnetic properties of the resultant magnet. Japanese Patent Laid-Open No. 59-219404 further teaches to convert the by-produced oxides to water-soluble hydroxides such as Ca(OH).sub.2, Mg(OH).sub.2, etc. by reacting them with water, and wash away the hydroxides. However, as known in the art, this reaction proceeds with vigorous heat generation. Therefore, the surface of the coarse alloy powder is likely to be oxidized during the washing process, thereby increasing the oxygen content of the final coarse alloy powder. When such a coarse alloy powder having a high oxygen content is made into a sintered magnet by pulverizing the coarse alloy powder into fine powder by a usual jet milling method, ball milling method or attritor milling method, dry-compacting the fine powder into a green body, and sintering the green body, the resultant sintered magnet has an oxygen content higher than that of a sintered magnet made of an alloy powder from an ingot prepared by a melting/casting method, thereby deteriorating magnetic properties, particularly reducing a coercive force.
With such a disadvantage, the use of the coarse alloy powder prepared by the reductive diffusion method has been limited. In some case, the coarse alloy powder prepared by the reductive diffusion method has been used in combination with the expensive alloy powder prepared by the melting/casting method. However, this cannot reduce largely the production cost of the rare earth permanent magnet. Thus, the advantage of the cheap coarse alloy powder prepared by the reductive diffusion method has not been sufficiently utilized in practice.